How do I uninstall Google Cloud SDK in Windows

On the latest SDK, Under "C:\Program Files (x86)\Google\Cloud SDK" you should find an uninstaller (uninstaller.exe). Just execute it.

I have also been struggling with this.

As far as I can tell, the Google App Engine SDK Windows installer installs a whole bunch of binaries into C:\Program Files\Google\Cloud SDK, installs Python 2 (2.7.6 as of this post's writing), creates a CLOUDSDK_PYTHON environment variable, and updates your PATH variable to reference the Python and Cloud SDK locations.

The Cloud SDK Core Command Line Tools (gcloud) can assist you with removing most of the binaries. It's been suggested elsewhere that these binaries can just be deleted, but I uninstalled them one-by-one, just to be safe. To do this (taken from L.H's post here), launch the Google Cloud SDK Shell from your Start menu, and run gcloud components list to see all the SDK components. Each can be removed by running gcloud components remove [COMPONENT_ID]. I removed all of the Individual Components before removing the Cloud SDK Core Command Line Tools Package.

Python includes an actual Windows uninstaller with its installation, you you can just remove it as normal via Add/Remove Programs.

Then, you can just Remove the Google Cloud SDK-related Environment Variables and the references in your PATH variable.